1. Field of the Invention
The present invention relates to digital video signal recording system and apparatus for recording digital video signals on a magnetic tape. More particularly, the present invention relates to a digital video signal recording system and apparatus operated both in an ordinary recording mode and in a long recording mode which enables recording for N (N: an integer) times as long hours as those in an ordinary recording mode.
2. Description of the Related Art
As an apparatus for recording picture information and sound information on a magnetic tape, an analog video tape recorder (VTR) such as a VHS VTR is conventionally known.
FIG. 10 schematically shows a general magnetic recording/reproducing means in a VTR.
In FIG. 10, a magnetic tape 1, a rotary drum 2 and rotary heads 3 and 4 are shown.
The magnetic tape 1 travels in parallel with the plane the drawing, while the rotary drum 2 rotates in the state being inclined at a predetermined angle with respect to the direction vertical to the plane of the drawing.
The rotary heads 3 and 4 are provided on the periphery of the rotary drum 2 in such a manner as to come into contact with the magnetic tape 1. Since the magnetic tape 1 is wound half around the periphery of the rotary drum 2, the magnetic tape 1 is scanned with the rotary heads 3 and 4 in an oblique direction. Consequently, the area of the magnet-tape 1 scanned in one cycle, which is called a track, is situated obliquely relative to the longitudinal direction of the magnetic tape 1.
the rotary drum 2 is rotated at a rate of about 1,800 rpm so as to record analog video signals for 1 field per track recording is carried out at a speed of 1 field/about 60 sec.
A VTR operated both in an ordinary recording mode and in a long recording mode is known. FIGS. 11 and 12 show the patterns of the tracks T1 and T2 in such a VTR in an ordinary recording mode and a long recording mode, respectively. In the case of a VHS system, the widths of the tracks T1, T2, namely the track pitches are about 58 .mu.m and 19 .mu.m in an ordinary recording mode and a long recording mode, respectively.
FIGS. 11 and 12, a belt-like control track (hereinunder referred to as "CT") 1a is provided in the longitudinal direction of the magnetic tape 1.
If the video signal recorded on the magnetic tape 1 is an NTSC video signal, a signal having a frequency of 30 Hz is recorded as a CT signal. The CT signal is a signal for controlling the travelling speed of the magnetic tape 1 by a servomechanism.
In order to realize the track patterns in both ordinary recording mode and long recording mode by one apparatus, each of the rotary heads 3 and 4 is composed of heads 3a, 3b and 4a, 4b, respectively, which have different scanning track widths from each other, (each of the rotary heads 3 and 4 is composed of what is called a combination head), as shown in FIG. 10 and the travelling speed of the magnetic tape 1 is changed in accordance with the mode.
For example, in a long playing mode for recording for N times as long hours as those in an ordinary recording mode, the travelling speed of the magnetic tape 1 is reduced to 1/N. In this case, as shown in FIG. 13, the period in which the CT signal is situated on the magnetic tape 1 in a long recording mode is reduced to 1/N of the period in an ordinary recording mode.
In reproducing the video signals recorded on the tracks T1 and T2, the CT signals are simultaneously reproduced from the CT 1a. If the travelling speed of the magnetic tape 1 is controlled by a servomechanism so that the reproduced CT signal has a frequency of 30 Hz (the frequency of a CT signal recorded in an ordinary recording mode) both in an ordinary recording mode and in a long recording mode, it is possible to reproduce the video signals.
In this way, recording/reproducing is conventionally possible both in an ordinary recording mode and in a long recording mode. However, the quality of the video signal reproduced in a long recording mode is inferior to that ordinary recording mode, because the track pitch is smaller. For this reason, a long recording mode is conventionally adopted when the reproduced video signals are not specially required to have a high quality or when long recording is necessary.
The above explanation is as to a system for recording analog signals on a magnetic tape. When the principle of this system is applied to recording of digital video signals on a magnetic tape, the following problems are produced.
a) In recording a digital video signal, it is necessary to keep the areal recording density and the linear recording density at the optimum values in order to secure a reproducing output. However, the areal recording density becomes small when the track pitch is small. Therefore, if the track pitch is reduced in a long recording mode, it is impossible to obtain an adequate reproducing output as in recording analog video signals.
b) On the other hand, if the number of revolutions of the rotary drum is reduced instead of reducing the track pitch, the relative speed between the rotary head and the magnetic tape greatly changes, thereby changing the electromagnetic transducing characteristics of the rotary head. In this case, it is also difficult to keep the good quality of the reproduced video signals.